A so-called “low dimensional quantum structure”, in which a microstructure of a semiconductor having a narrow band gap is surrounded in a two-dimensional or three-dimensional manner by a semiconductor having a wide band gap, is regarded as promising for achieving enhanced functions and performances of optical devices and electronic devices, and recently has been attracting an increasing interest as a key for the future development of the optical and electronic industries. In particular, since a quantum dot as a three-dimensional quantum enclosing structure exhibits a remarkable quantum effect in a wide variety of applications due to the sharpness of a state density based on the strong electron-enclosing effect, realization of the quantum dot is anticipated as a basic structure of the optical and electronic devices having functions and performances superior to those of the prior art.
As for a technique for forming these microstructures, there is a lithography technique utilizing an electron beam, an ion beam, or an STM needle. In recent years, a fine patterning up to 100 nm or less has become possible. However, these methods have such a disadvantage that a manufacturing cost is still high, and it takes a lot of time to carry out the processing. In addition, in order to further increase the enclosing effect, a structure having a size on the order of even smaller than 100 nm is required.
In order to solve the above-mentioned problems, there has been proposed a method in which a substrate is selectively etched using a mask having a microstructure to form a microstructured material. Since a large number of microstructured materials can be formed at a time by the method using a mask, this method is very advantageous in terms of processing time.
In JP 11-112099 A, there is disclosed a method in which first of all, a mask material is deposited on a substrate with a porous material called a trough hole membrance having a plurality of through holes as a mask, a pattern of dots made of the mask material is formed, and the substrate is selectively etched with the dot pattern as a mask to form minute projections on a surface of the substrate.
In addition, in Journal of Applied Physics, Vol. 91, No. 9, 6057 (2002), there is reported a method in which a cluster of gold is formed on a surface of a substrate by utilizing nucleus portions of micelle formed from a diblock copolymer, and a pillar-like structured material is manufactured with the cluster as a mask. In this case, it is described that a size of the mask, i.e., the cluster can be regulated by an amount of metallic salt dissolved in a diblock polymer solution, and intervals of arrangement can be regulated by a molecular weight of a hydrophobic portion of the diblock polymer.
In these methods using a mask, it can be said that a size of a microstructured material finally formed, and intervals of arrangement are substantially determined by a structure of the mask.
In JP 11-112099 A as well, a structure of the minute projections is determined by a structure of the through hole membrane serving as the first mask. The through hole membrane described in above-mentioned JP 11-112099 A is formed such that another substrate having projections arranged at desired intervals is pressed against an aluminium substrate to form a minute depression pattern on the aluminium substrate, and next, the aluminium substrate is subjected to anodic oxidation in an acid electrolytic solution to thereby form holes from the minute depression portions. Accordingly, arrangement intervals of the minute depressions can not be made equal to or smaller than the original arrangement intervals of the projections on the other substrate, and hence it is conceivable that the practical limit of the arrangement intervals is of the order of several nanometers. In addition, although a hole diameter can be increased within a range not exceeding the arrangement intervals through an after-treatment, it is difficult to make the hole diameter smaller. Hence, the hole diameter is substantially of the order of several tens nano-meters in many cases.
However, it is said that if a size of a microstructure of a semiconductor is made about equal to or smaller than 20 nm, then a distribution of energies of the electrons or holes within the structure can be made very narrow. For example, if a microstructure called a quantum fine line or a quantum dot is applied to a semiconductor laser, then it is possible to realize a semiconductor laser having an extremely low threshold current. Accordingly, in order to realize such a semiconductor laser, there is required a technique for uniformly and densely forming a structure having a shape of a size of the order of even smaller than several tens nano-meters mentioned above.
In addition, application of such a microstructured material to a single electron device such as a single electron transistor or a single electron memory is also anticipated. However, in many cases, such a microstructured material exhibits its unique property such as a quantum size effect only when its size becomes smaller than 10 nm. Accordingly, from the viewpoint of application to the single electron device as well, the realization of an ultra-micro structure is desired.
On the other hand, according to the report of Journal of Applied Physics, Vol. 91, No. 9, 6057 (2002), a pillar-like structured material having a diameter equal to or smaller than 10 nm becomes possible. However, the arrangement intervals are regulated by a size of the micelle formed from the diblock polymer, and hence are about 100 nm. For stable formation of the micelle, it is necessary to stably separate a hydrophobic portion and a hydrophilic portion of the diblock polymer. In order to attain this, a certain chain length is required for the hydrophobic portion and the hydrophilic portion within the polymer. Accordingly, there is actually a limit to shortening of the arrangement intervals by shortening of a chain length of the hydrophobic portion. Hence, it can be said that problems remain in achieving higher density as mentioned above.
The present invention has been made in the light of the above-mentioned problems, and it is therefore an object of the present invention to provide a method of forming, on a substrate, columnar portions of a columnar structured material each having an extremely minute size at minute intervals at a low cost and in a short period of time, and a columnar structured material formed by utilizing the manufacturing method.